The injection of additives is quite particularly of interest in the case of diesel engines, and additives can be envisaged which are intended for reducing the formation of soot during the burning of diesel fuel and/or lowering the self-ignition temperature of the soot, or more generally additives intended for lubricating the liquid fuel, or alternatively various organometallic compounds.
The injection of additives has long been used by petroleum plants, when the fuel is still stored in the storage vessel, so that the additive is ipso facto incorporated into the fuel when it is delivered to the petrol stations.
In this case, it is a technique which does not require high precision in the metering, because hugh volumes are dealt with.
For several years, motor vehicle manufacturers have sought to develop techniques for automatic injection of additives into the fuel tank of a motor vehicle or, from an on-board additive tank.
In this case, the injection method must be as accurate as possible, in order to have a concentration held at the value desired, and the associated device must be flexible, compact and robust. The additive/fuel mixture must furthermore be as homogenous as possible.
One advantageous technique is illustrated in document U.S. Pat. No. 4,621,593.
In this document, an immersed solenoid pump is used for withdrawing some of the additive from the additive tank, downstream of which is provided a pipe arriving at the top of the fuel tank. When refuelling, the corresponding level variation is detected by a float mounted at the end of a pivoting arm, the articulation of which carries a rheostat system, which allows an on-board computer to deduce from this resistance the quantity of additive which it is necessary to withdraw, and accordingly to control the immersed solenoid pump.
The injection is thus carried out at low pressure (of the order of 5.10.sup.5 Pa), and the additive is released onto the surface of the fuel, directly leaving through the associated pipe. Because of this, the mixture is never truly homogeneous: the additive injected generally has a different density from that of the fuel, so that this additive tends to remain in the form of a layer (at the top or at the bottom of the tank), so that the fuel pumped from the bottom of the tank by the injection pump of the engine is not certain to have the desired concentration. Furthermore, the computer is not designed to allow any subsequent operation: the concentration is determined once only, which leads to a certain inflexibility when it is desired in particular to use the same system with other additives.
The document DE-C-3,626,419 illustrates a system similar to the preceding one, but described more briefly. Here again, the additive injected is being released at the top of the fuel tank, through a bent tube whose other end emerges directly into the filler tube. Because of this, the accuracy and the homogeneity of the mixing are necessarily limited.
It has also been proposed, in order to improve the precision of the metering obtained with conventional pump systems, to use a withdrawal syringe, whose plunger is directly coupled to a float arm associated with the level of fuel in the tank. Such a technique is in practice very difficult to implement. Furthermore, the aforementioned drawbacks relating to the low degree of homogeneity of the mixture and the inflexibility of the system as regards the concentration of the mixing are again encountered.
The aforementioned techniques involve the injection of additive when the vehicle is stationary and turned off, which is in general preferable for safety reasons. Furthermore, the driver can then easily check that the injection is proceeding correctly, and possibly be alerted in time in the event of a difficulty (insufficient additive level, for example).
Injection techniques operating when the vehicle is running have also been proposed.
In one case, it was sought to carry out automatic injection of additive as a function of the variation in level following refuelling (as illustrated in document FR-A-2,668,203), whereas in other cases, it was sought to carry out injection of additive as a function of the fuel consumption in the engine (as illustrated in document EP-A-0,269,228).
The technique illustrated in document FR-A-2,668,203 provides for the injection of additive to be carried out through a duct connecting the tank to the delivery part of the fuel injection pump. The injector thus connected up to the return duct of the fuel injection pump carries out injection at constant pressure for a calculated time, by correspondingly controlling an associated rotary pump. A pressure limiter thus ensures that the additive flow rate is constant. The accuracy of the metering is therefore necessarily limited, even if the operating times of the pump are calculated with precision.
Furthermore, such injection carried out in the return duct does not make it easy to obtain accurate metering, and therefore necessarily assumes that the vehicle is running, so that the injection pump delivers the quantity of additive injected, which would otherwise flow slowly through this duct. It is indicated in this document that the operation of the fuel injection pump provides a stirring effect which favours the obtaining of a more homogeneous mixture, but this effect is necessarily limited in as much as the additive injection is carried out under low pressures (in general not more that 7.10.sup.5 Pa). Furthermore, this technique still remains inflexible as regards the proportion chosen for the additive, the metering being essentially fixed.
The technique illustrated in document EP-A-0,269,228 is for its part highly complex to implement, and the device is bulky. It is not however absolutely necessary to seek "real time" additive injection, as a function of the fuel consumption in the engine.